CONDUCTION AND RADIATION. 537 



which the planets of the solar system revolve. In 

 his Memoir, published in 1827 17 , Fourier states that 

 he conceives it to follow from his principles, that 

 these planetary spaces are not absolutely cold, but 

 have a " proper heat" independent of the sun and of 

 the planets. If there were not such a heat, the cold 

 of the polar regions would be much more intense 

 than it is, and the alternations of cold and warmth, 

 arising from the influence of the sun, would be far 

 more extreme and sudden than we find them. As 

 the cause of this heat in the planetary spaces, he 

 assigns the radiation of the innumerable stars which 

 are scattered through the universe. 



Fourier says 18 , "We conclude from these various 

 remarks, and principally from the mathematical 

 examination of the question," that this is so. I am 

 not aware that the mathematical calculation which 

 bears peculiarly upon this point has anywhere been 

 published. But it is worth notice, that Svanberg 

 has been led 19 to the opinion of the same tempera- 

 ture in these spaces which Fourier had adopted (50 

 centigrade below zero), by an entirely different 

 course of reasoning, founded on the relation of the 

 atmosphere to heat. 



In speaking of this subject, I have been led to 

 notice incomplete and perhaps doubtful applica- 

 tions of the mathematical doctrine of conduction 

 and radiation. But these may at least serve to show 



17 Mem. I nsl. tom.vii. p. 580. '" Ib. p. 581. 



!) Berzcl. Jahrex In'r/r/if, xi p. 50. 



